The present invention relates to a method for manufacturing thin films, and in particular to a process of manufacturing thin films using the combined RF (Radio Frequency)-DC (Direct Current) magnetron sputtering method. In this invention, the supply of power is improved to prevent a tracking arc from being produced and to ensure the consistent manufacture of thin films.
Sputtering is an indispensable technique for depositing films typical used in electronic device manufacturing processes, and is widely known as a dry process technique with a wide range of applications. Sputtering is a method in which a rare gas such as argon is introduced into a vacuum container. Direct current (DC) or radio frequency (RE) power is supplied to a cathode including a target to produce a glow discharge and thereby deposit a film. The former is referred to as DC sputtering, while the latter is referred to as RF sputtering.
FIG. 6 schematically depicts the potential distribution between the cathode and anode (ground potential) during discharge. Vp is the plasma time-averaged potential, and Vt is the cathode surface (that is, target surface) time-averaged potential. As shown in the figure, the resulting Vt of glow discharge is a negative potential with respect to the Vp. As a result of the difference in potential (Vpxe2x88x92Vt: referred to as self bias in the case of RF sputtering), positive ions such as those of accelerated argon collide with the surface of the target, which is attached to the cathode, and the target is sputtered. Sputtered particles from the target build up on a treatment piece (substrate) facing the target. When a mixed gas of a rare gas such as argon and a reactive gas such as O2 or N2 is introduced into the vacuum container at this time, the reaction product of the target material and this reactive gas builds up on the substrate.
The aforementioned sputtering includes magnetron sputtering, where magnets are placed behind the target to increase the plasma density around the target surface, resulting in more rapid film deposition. Magnetron sputtering includes RF magnetron sputtering using RF power, and DC magnetron sputtering using DC power. Both are widely used methods for depositing films during mass production.
There has been considerable progress in electronic devices recently, resulting in the need to develop techniques for improving thin film properties, including techniques for depositing films by magnetron sputtering. A factor which adversely affects the properties of thin films, when films are deposited by sputtering, is that the thin film is damaged by the impact of high energy particles on the substrate. The energy of these high energy particles is caused by differences in potential mainly arising on the front surface of the target. The difference in potential must be reduced to obtain high quality thin films. In the case of RF and DC magnetron sputtering, the Vt given in FIG. 6 is determined by the container configuration, pressure, magnetic field intensity, and the conditions of the power supply.
Another method is combined RF-DC magnetron sputtering, where RF and DC power are simultaneously supplied to the target to cause sputtering. The Vt can be controlled by the voltage of the DC power source supplying the DC power during combined RF-DC magnetron sputtering. A high quality thin film can thus be manufactured because the difference in potential produced on the front surface of the target can be reduced by increasing Vt during combined RF-DC magnetron sputtering.
However, one problem with sputtering is that an abnormal discharge is produced on the target or on the surface of other parts inside the vacuum container. More specifically when an ITO transparent conductive film consisting of In (indium), Sn (tin), and O (oxygen) is formed on a substrate using an In and Sn oxide as a target by magnetron sputtering, or when a GeSbTe phase change type of recording film is deposited on a substrate using a Ge (germanium), Sb (antimony), and Te (tellurium) compound as the target (general composition: Ge2Sb2Te5), an abnormal discharge with rotating arcing is produced in portions on the target where the magnetic field perpendicular to the target surface is zero (that is, the portion where the target is mostly etched). Such an abnormal discharge is referred to as a xe2x80x9ctracking arcxe2x80x9d here. A tracking arc is not unusual even when using combined RF-DC magnetron sputtering which capable of manufacturing high quality thin films.
When a tracking arc is produced, the discharge impedance changes, and power cannot be supplied efficiently to the target. As a result, the film is formed at a lower rate, or films cannot be completely deposited. In some cases, a tracking arc results in the deposition of films with completely different properties.
A tracking arc also causes dust particles to be produced. When such dust particles adhere to the substrate, defects and product imperfections result.
A tracking arc is less readily produced when the magnetic field intensity at the target surface is weakened, when the film depositing pressure is lowered, and when the power supplied is reduced. However, a tracking arc cannot be completely suppressed by such methods. Such methods also cause production problems by lowering the film deposition rate.
An object of the present invention is to provide a thin film manufacturing method which suppresses a tracking arc and allows thin films to be consistently manufactured when such thin films are manufactured by combined RF-DC magnetron sputtering.
Findings leading to the structure of the present invention as a means to achieving the aforementioned objectives will be discussed first.
The mechanism and causes of a tracking arc are not currently understood. The inventors conducted painstaking research to remedy the problem of a tracking arc in magnetron sputtering. As a result, they arrived at the following considerations on the mechanism and causes of a tracking arc.
It has been reported that in processes featuring the use of RF discharge, negatively charged clusters grow in the interface between the plasma and cathode sheath in the course of discharge. This is discussed, for example in the article by Shiratani et al in J. Appl. Phys. 79(1), (January 1996), pp. 104-109. This is attributed to cohesion with positively ionized free particles as a result of collision between negatively ionized free particles and high energy electrons of the g-electrons released from the cathode. The clusters which increase as a result of particle cohesion are negatively charged because of the increase in the collision area with the electrons. The negatively charged clusters and positive ions cohere further, and the clusters grow. The growth of the negatively charged clusters are considered a cause of the aforementioned tracking arc.
In the case of magnetron sputtering, most g-electrons and sputter particles are released in a part where target erosion is deepest, and the g-electrons are trapped in the magnetic field produced by the magnets. Extremely large clusters thus continue to be negatively charged and grow over the part where target erosion is deepest. Once such cluster growth and xe2x80x9ccharging upxe2x80x9d passes a certain level, an arc is produced between the cluster past the level first and the target. The target is ablated by this arcing, resulting in a plume (fuming). Pressure in the plume is high, and arcing persists through the concentration of discharged power. At this time, the plume acts as a current path, and the arc rotates in the part of deepest erosion because it behaves as a conductor through which flows the current moving in the magnetic field. The aforementioned tracking arc is an arc with such properties.
Because of the above, time is needed for cluster growth in the case of materials (such as ITO consisting of In and Sn oxides, or Ge2Sb2Te5, a compound of Ge, Sb, and Te) which have a high g-electron emission coefficient, are readily ablated, and tend to produce a plume. This is evident in light of the fact that a tracking arc was not produced at the same time that power began to be supplied (that is, at the initiation of discharge) during research.
Research demonstrated that no tracking arc was produced when the power supply was low. This suggests that plasma density is related to the production of a tracking arc. When the power supply is low, the plasma density is low, and the positive ion density therein is therefore low, allowing the growth of clusters to be suppressed. When the plasma density is low at the same time as this, the plasma-based shield weakens, and the negatively charged clusters are scattered by the electrostatic repulsion force due to mutual charge. Low power supply produces no tracking arc.
In the present invention, a time is established to suppress the growth of clusters and scatter them at a stage prior to the production of a tracking arc, thereby solving this problem.
The discharge should preferably be stopped to suppress the growth of clusters and scatter them. To suppress the growth of clusters and scatter them, the discharge need not necessarily be completely stopped. However, the power supply can be lowered to a certain level to lower the plasma density, allowing the cluster growth to be suppressed and scattered. A tracking arc can thus be prevented simply by lowering the power supply before the tracking arc is produced.
Based on the aforementioned findings, the present invention was constructed in the following manner.
In a first combined RF-DC magnetron sputtering method, the supply of the RF power and DC power to the aforementioned target is simultaneously and periodically stopped, and the aforementioned RF and DC power is supplied for a shorter time than the time needed for a tracking arc to be produced. That is, the supply and interruption of RF and DC power is synchronized, and power is thus intermittently supplied to the target. In the combined RF-DC magnetron sputtering method, magnets are placed behind the target, RF and DC power is supplied simultaneously to the target to produce a plasma, and sputtering is used to manufacture a thin film on a substrate facing the target.
In the first embodiment of the present invention, the period for suppressing the growth of clusters and scattering them is repeated by synchronizing and periodically managing the supply and interruption of RF and DC power to the target. The time for supplying power at this time is shorter than the time needed for the tracking arc to be produced, allowing such tracking arcs to be prevented.
Here, thin films are usually deposited by sputtering at a film forming pressure of about 0.1 to 1 Pa. When the initiation pressure of discharge is lower than the film depositing pressure, the supply of power may be discontinued for a longer time during the aforementioned intermittent supply of power to the target, making discharge possible when power is subsequently resumed, even when the discharge is completely stopped. However, depending on the device, for example, when the cathode (target) is of a small size, the initiation pressure of discharge is sometimes higher than the film depositing pressure. In such cases, when the power supply is interrupted for a longer time, and the discharge is completely interrupted, discharge can no longer be started at the film depositing pressure, even when power is then resumed. Accordingly, when the initiation pressure of discharge is thus higher than the film depositing pressure, it is better to avoid complete interruption of discharge without stopping the supply of power.
In a second combined RF-DC magnetron sputtering method, a period is established for simultaneously and periodically reducing the RF power and DC power supplied to the target, and the time in which the supplied power is supplied without being reduced is shorter than the time needed for a tracking arc to be produced.
In the second embodiment of the present invention, a period is set in order to lower, not completely stop, the supply of power to the target as in the first embodiment. The discharge is therefore not stopped, allowing discharge to be maintained at the film depositing pressure even when the initiation pressure of discharge is higher than the film depositing pressure. As is evident in light of the results of the research described above, lowering the power supplied to the target allows the plasma density to be reduced, and this allows the growth of clusters to be suppressed and scattered. The time when the power is supplied without being reduced is shorter than the time needed for a tracking arc to be produced, preventing a tracking arc in this method as well.
In a third combined RF-DC magnetron sputtering method, a constant voltage regulated power source is used for the DC power source supplying the DC power to the aforementioned target, and the set voltage of the constant voltage regulated power source is adjusted to no more than the voltage needed to maintain discharge (the lower absolute value) during discharge by DC power alone. Also, the supply of the RF power to the aforementioned target is periodically stopped, and the RF power supply time is shorter than the time needed for a tracking arc to be produced.
In the third embodiment of the present invention, the set voltage of the DC power is constant at no more than the voltage needed to maintain discharge (the lower absolute value) during discharge by DC power alone. The RF power supplied to the target is periodically stopped based on the constant DC voltage. If the set voltage of the constant voltage regulated power source is set to no more than the voltage needed to maintain discharge (the lower absolute value) during discharge by DC power alone, discharge takes place during the period in which RF power is supplied. Power is supplied from the constant voltage regulated power source as well, but no discharge can be maintained during the period in which no RF power is supplied, and no power is supplied from the constant voltage regulated power supply either. By periodically stopping the supply of RF power, power is supplied to the target for discharge, and the supply of power is stopped to stop discharge. A period can be set to suppress the growth of clusters and scatter them. The RE power is supplied for a shorter time than the time needed to produce a tracking arc so as to prevent the tracking arc from being produced in the same manner as in the first embodiment.
A fourth combined RF-DC magnetron sputtering method is based on the same assumptions as in the first embodiment of the present invention. A constant voltage regulated power source is used for the DC power source supplying the DC power to the target, and the set voltage of the constant voltage regulated power source is adjusted to no more than the voltage needed to maintain discharge (the lower absolute value) during discharge by DC power alone. A period is established for periodically reducing the RF power supplied to the target, and the time in which the aforementioned RF power is supplied without being reduced is shorter than the time needed for a tracking arc to be produced.
In the fourth embodiment of the present invention, the set voltage of the constant voltage regulated power source is set to no more than the voltage needed to maintain discharge (the lower absolute value) during discharge by DC power alone, so as to reduce the supply of RF power. This allows the plasma density to be reduced, and allows the growth of clusters to be suppressed and scattered. The period for suppressing and scattering cluster growth is set periodically, and the time when the RF power is supplied without being reduced is shorter than the time needed for a tracking arc to be produced. A tracking arc can be prevented by this method as well.
In another aspect of the invention for combined RF-DC magnetron sputtering method, the time for stopping the power supply or the time for reducing the power supply is at least 1 millisec. When the power supplied to the target is interrupted, the plasma is not instantaneously extinguished, but the density is gradually reduced. In the case of the commonly used sputtering gas argon, the time until the plasma density reaches almost zero after the supply of power has been stopped is about 1 msec. Even when the power supply has been reduced, it takes about 1 msec to reach a plasma density where the discharger is stable at the reduced power. In this embodiment of the present invention, a sufficient reduction in plasma density can be achieved by adjusting the time for stopping or reducing the power supply to at least 1 msec. This allows the growth of clusters to be suppressed and scattered, so that a tracking arc can be prevented from being produced.